The invention relates to a safety belt pretensioner in a restraining system for vehicle occupants. In a safety belt pretensioner known from German patent specification 3,249,729, a rotatable reel has a slot for the passage of the belt webbing. The reel is provided at its outer periphery with a toothing frame which is adapted to be mounted on the vehicle bodywork comprises a first wall having a toothing along which the toothing of the reel can roll in meshing engagement. A linear drive which is activatable by a collision of the vehicle transmits linear movement to the reel for conversion into a rotary movement of the reel. This pretensioner has the advantage that a conventional belt retractor can be employed in the safety belt restraining system. Integration into existing systems is relatively easy because the webbing path remains unchanged.